1. Field of the Invention
This invention relates to bearing assemblies for supporting rotating shafts and, more particularly, to an apparatus for controlling vibration in the event of abnormally high shaft unbalance.
2. Description of the Prior Art
Rotating shafts, and particularly rotating shafts in gas turbine engines, can become abnormally unbalanced while operating. For example, a high pressure turbine rotor in a gas turbine engine will become abnormally unbalanced in the event that a turbine blade is lost. The combination of abnormally high unbalance and a critical speed within the operating range of the rotating turbine shaft could result in potentially damaging vibration forces as well as excessive shaft deflections which may result in damaging rubs.
Prior art bearing assemblies control the shaft vibration characteristics by means of critical speed adjustments and damping. The critical speed is usually controlled by adjusting the flexibility of the various members which comprise the engine system, e.g. the rotating shaft itself, the bearing, support structure, etc. The vibration response at the critical speeds is often suppressed by means of a single squeeze film damper which usually comprises a quantity of oil contained within a small radial gap.
Although the prior art bearing assemblies are generally adequate for achieving smoothness in a moderately unbalanced system during normal operation, they are generally inadequate for an abnormally high unbalanced system. For example, when a prior art bearing assembly is employed with a high pressure turbine rotor in a gas turbine engine, and in the event a turbine blade is lost, the small gap associated with a single squeeze film damper provides a nonlinear hard spring response which tends to raise the critical frequency and negate the advantages derived from the damping. Increasing the size of the radial gap results in a softer spring rate, but the damping force is significantly reduced, tending to result in damaging rubs.